<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>链式调用</title>
</head>

<body>
    <script>
        function ajax(url) {
            return new Promise((resolve, reject) => {
                let xhr = new XMLHttpRequest()
                xhr.open('get', url)//链接
                xhr.send()//发送资源
                xhr.onreadystatechange = function () {
                    console.log(xhr)
                    if (xhr.readyState === 4) {
                        if (xhr.status >= 200 && xhr.status < 300) {
                            xhr.responseText //json 字符串
                            resolve(res)
                        } else {
                            reject(xhr.responseText)
                        }
                    }
                }

            })

        }
        ajax('./01.json')
            .then((res) => {
                console.log(res);
                return ajax('./02.json')
            })
            .then((res) => {
                console.log(res);
            })
            .catch((err) => {
                console.log(err);
            })
    </script>
</body>

</html>